High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539, incorporated herein by reference. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly (ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly(1,4-butylene terephthalate), becuase of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
It has now been discovered that reinforced blends of poly(1,4-butylene terephthalate) and poly(ethylene terephthalate) can be impact modified with the addition of a polyolefin or olefin based copolymer resin. The polyolefinic resin is easily dispersed into the polyester blend and results is greatly improved impact strength in the molded article in surprisingly small amounts. The heat distortion temperature is also enhanced.